Architect Frank Gehry is working with city officials to draft a new master plan for the redevelopment of the Los Angeles River, bringing the avant-garde sensibilities of one of the world's best-known artistic celebrities to the struggle to remake 51 miles of the Los Angeles Basin's largely desolate central waterway.

Gehry's involvement marks a potential turning point in a decades-long struggle to reinvent the river and its concrete-lined banks. But his plan is getting a cold reception from some of the activists who helped draw attention to the cause.

Some express concern about the secrecy surrounding Gehry's work and the lack of public input on what could be a far-reaching blueprint for the river's future. They also warn that announcing a new direction for river redevelopment might imperil federal funding for a $1.4-billion, 11-mile restoration project in northeast Los Angeles and downtown.

Those concerns came to a head this week, when Friends of the Los Angeles River, the nonprofit group led by Lewis MacAdams, sent a letter to the River Revitalization Corp. refusing to endorse the Gehry effort. The letter also said the river environmental group would not take part in a news conference announcing the architect's plan.